The invention relates to a process for tobacco expansion. More specifically, the invention relates to a process for expanding tobacco to reduce its bulk density and thereby increase its volume and filling capacity. The process is especially suitable for treating cigarette cut filler.
In the past two decades, tobacco expansion processes have become an important part of the cigarette manufacturing process. Tobacco expansion processes are used to restore tobacco bulk density and/or volume which are lost during curing and storage of tobacco leaf. In addition, expanded tobacco is an important component of many low tar and ultra low tar cigarettes.
Commercially significant tobacco expansion processes are described in U.S. Pat. No. 3,524,451 to Fredrickson and U.S. Pat. No. 3,524,452 to Moser et al. These patents describe processes wherein tobacco is contacted with a volatile organic impregnant and then heated by rapidly passing a stream of hot gas in contact with the impregnated tobacco to volatilize the impregnant and expand the tobacco. A variation of these processes is described in U.S. Pat. No. 3,683,937 to Fredrickson et al. which discloses a tobacco expansion process wherein tobacco is impregnated with a volatile organic compound in the vapor state and in the absence of any liquid or solid phase. The impregnated tobacco is expanded either by heating or rapidly reducing pressure. Heat can be applied through a stream of hot gas or through microwave treatment.
Following development and commercialization of the tobacco expansion processes described above, extensive and continuing efforts have been directed to the identification of specific expansion agents and processes for expansion of tobacco. For example, U.S. Pat. No. 4,235,250 to Utsch; U.S. Pat. No. 4,258,729 to Burde et al. and U.S. Pat. No. 4,336,814 to Sykes et al., among others, disclose the use of carbon dioxide for expanding tobacco. In these and related processes, carbon dioxide, either in gas or in liquid form, is contacted with tobacco to impregnate tobacco and thereafter the carbon dioxide-impregnated tobacco is subjected to rapid heating conditions to volatilize the carbon dioxide and thereby expand the tobacco. Carbon dioxide is a substantial component of the atmosphere and is readily available. Nevertheless, in the carbon dioxide tobacco expansion processes, it is typically necessary to heat the tobacco excessively in order to achieve substantial stable expansion which can result in harm to the tobacco flavor and the generation of excessive amounts of tobacco fines. In addition, those commercially available processes which use liquid carbon dioxide to impregnate tobacco result in impregnated tobacco in the form of solid blocks of tobacco containing dry ice which must be broken up prior to heat treatment, thereby harming the tobacco and increasing the complexity of the process.
U.S. Pat. No. 4,461,310 to Zeihn and U.S. Pat. No. 4,289,148 to Zeihn describe the expansion of tobacco employing supercritical nitrogen or argon impregnation of tobacco. These gases are removed from the tobacco during a rapid pressure reduction and the tobacco is expanded by exposure to heated gas or microwave. These processes require treatment of tobacco at pressures in excess of 2,000 or 4,000 psi up to above 10,000 psi in order to achieve substantial tobacco expansion.
U.S. Pat. No. 4,531,529 to White et al. describes a process for increasing the filling capacity of tobacco wherein the tobacco is impregnated with a low-boiling and highly volatile expansion agent such as a normally gaseous halocarbon or hydrocarbon at process conditions above or near the critical pressure and temperature of the expansion agent. The pressure is quickly released to atmospheric so that the tobacco expands without the necessity of a heating step to either expand the tobacco or to fix the tobacco in the expanded condition.
Various processes have been disclosed for the microwave treatment of tobacco to provide tobacco expansion in U.S. Pat. No. 3,765,425 to Stungis et al., U.S. Pat. No. 3,842,846 to Laszio et al. and U.S. Pat. No. 3,881,498 to Wochnowski, among others. In the Stungis et al. disclosure, tobacco is treated to increase its moisture level or to impregnate the tobacco with an organic expansion agent which absorbs microwaves. Alternatively, the tobacco is treated with an organic expansion agent which does not absorb microwaves, in combination with water and thereafter in any case, the tobacco is exposed to microwave energy to volatilize the moisture and/or organic expansion agent, resulting in tobacco expansion.
Numerous other compounds have been proposed or suggested for expanding tobacco including alkanes, alkenes, alcohols, aldehydes, ketones and ethers. In most instances, various practical problems are encountered however, such as the extraction of desirable flavors from the tobacco during the impregnation step and/or the expansion step; insufficient amount of tobacco expansion; non-uniformity of expansion; reactions between the expansion agent and various components in tobacco; adverse impact on tobacco processing equipment; high levels of retained residual in the final expanded tobacco; and/or hazards such as flammability associated with expansion agents.
There has thus continued to be a search for improvements in known tobacco expansion processes and for new and improved tobacco expansion processes and agents, in general. Yet despite the continuing efforts, commercial success in the field of tobacco expansion has been limited.